Electrochemical reductive coupling is an important type of carbon-carbon bond-forming reactions. A large variety of starting materials has been employed successfully. Substituted olefins are an important class of these compounds. They can hydrodimerize with themselves or couple with other compounds, such as carbonyl compounds.
An industrially important example of an electrohydrodimerization reaction is the electrosynthesis of adiponitrile, an important precursor of nylon-6,6 (M. M. Baizer, Chemtech 1980, 10, 161; D. E. Danly, AIChE Symposium Series 1981, 77, 39).
The cathodic surface of the electrochemical cell must have a cathodic potential sufficient for the electrochemical reduction of a substrate. The electrochemical reduction of the substrate, e.g., the olefinic compound, competes with the reduction of protons which are present in the electrolyte solution and also necessary for the electrosynthesis pathway. Successful reductive coupling requires that one substrate is reduced preferentially over the protons in the first step. Water is in many cases the preferred proton source. To gain good selectivities and yields, electrode materials with a high hydrogen overpotential are conventionally used, such as lead or mercury electrodes (M. F. Nielsen, J. H. P. Utley, in Organic Electrochemistry, 4th ed., 2001, 795, H. Lund, O. Hammerich, Eds., Marcel Dekker, New York).
S. M. Makarochkina and A. P. Tomilov (J. Gen. Chem. USSR 1974, 44, 2523) disclose that tertiary alcohols with various functional groups can be obtained by the reductive coupling of aliphatic ketones with activated olefins in a divided cell, utilizing mercury or graphite cathodes. Alkenes without electron-withdrawing groups, such as styrene, generally give poor coupling yields.
M. Nicolas and R. Pallaud (C. R. Acad. Sc. Paris 1967, 265, Série C, 1044) disclose the use of a mercury electrode in an aqueous electrolyte for the electrochemical reductive coupling of acetone and styrene, yielding 2-methyl-4-phenyl-2-butanol. While mercury cathodes may lead to increased yields, their use can be problematic, e.g. due to the ecologically troublesome accumulation of mercury-containing waste.